haskell: Phase 9 — error builtin raises with hk-error: prefix (+2 tests, 57/57)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/haskell-completeness.md

@@ -122,7 +122,11 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 ### Phase 9 — `error` / `undefined`
 
-- [ ] `error :: String -> a` — raises `(raise (list "hk-error" msg))` in SX.
+- [x] `error :: String -> a` — raises `(raise "hk-error: <msg>")` in SX.
+  _Plan amended:_ SX's `apply` rewrites unhandled list raises to a string
+  `"Unhandled exception: <serialized>"` before any user handler sees them, so
+  the tag has to live in a string prefix rather than as the head of a list.
+  Catchers use `(index-of e "hk-error: ")` to detect.
 - [ ] `undefined :: a` = `error "Prelude.undefined"`.
 - [ ] Partial functions emit proper error messages: `head []` →
   `"Prelude.head: empty list"`, `tail []` → `"Prelude.tail: empty list"`,
@@ -289,6 +293,18 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 9 `error :: String -> a` raises with `hk-error:` prefix:
+- Pre-existing `error` builtin was raising `"*** Exception: <msg>"` (GHC
+  console convention). Renamed prefix to `"hk-error: "` so the wrap-around
+  string SX's `apply` produces (`"Unhandled exception: \"hk-error: ...\""`)
+  contains a stable, searchable tag.
+- Investigation confirmed that the plan's intended `(raise (list "hk-error" msg))`
+  format is mangled by SX `apply` to a string. Plan note added; tests use
+  `index-of` substring matching against the wrapped string.
+- 2 new tests in `tests/eval.sx` (string and computed-message form). Suite
+  is 57/57. Other test suites unchanged (match 31/31, stdlib 48/48, derive
+  15/15, do-io 16/16, class 14/14).
+
 **2026-05-07** — Phase 8 conformance: `showadt.hs` + `showio.hs` (both 5/5):
 - `program-showadt.sx`: `deriving (Show)` on the classic `Expr = Lit | Add | Mul`
   recursive ADT; tests `print` on three nested expressions and inline `show`